Tegra: memctrl_v2: zero out NS Video memory carveout region

The video memory carveout has to be re-sized depending on the Video
content. This requires the NS world to send us new base/size values.
Before setting up the new region, we must zero out the previous memory
region, so that the video frames are not leaked to the outside world.

This patch adds the logic to zero out the previous memory carveout
region.

Change-Id: I471167ef7747154440df5c1a5e015fbeb69d9043
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>